The mixing of large quantities of inorganic fillers such as powdered aluminum hydroxide, powdered aluminum oxide, or powdered quart with silicone rubber compositions for use in applications requiring high-voltage electrical insulating characteristics, such as anode caps, plug boots, insulators, and flame-retardant electrical wiring, has been proposed. For example, Elliott, U.S. Pat. No. 3,965,065, teaches that a silicone rubber composition in which the ordinary electrical insulating characteristics are improved can be obtained by heating a mixture of a heat-curable silicone rubber composition and an aluminum hydrate for 30 minutes or longer at a temperature exceeding 100.degree. C. Furthermore, Hirai, et al., teach in Japanese Patent Application Kokai No. 4-209655 that a mixture consisting of (a) a polyorganosiloxane which forms an elastic material when heat-cured using an organic peroxide, (b) aluminum hydroxide and (c) an organosilane or organosiloxane which has phenyl groups, will form a silicone rubber composition with superior high-voltage electrical insulating characteristics.
The approach of improving electrical properties by the addition of inorganic fillers has several limitations. First, the silicone rubber compositions obtained by a high degree of filling with inorganic fillers such as aluminum hydroxide or aluminum hydrate can have extremely high viscosity. This makes them difficult to use in applications requiring fluidity, such as injection-molding. Moreover, such compositions also have a low mechanical strength. In some instances the high-voltage electrical insulating characteristics of the silicone rubber materials are not adequate. Furthermore, under conditions of harsh contamination or exposure to the elements, deterioration phenomena such as tracking and erosion occur due to high electrical stress, so that the high-voltage electrical insulating characteristics show a conspicuous deterioration.
The object of this invention is to provide a curable liquid silicone rubber composition which has an appropriate degree of fluidity prior to curing, which is superior in terms of moldability, and which forms silicone rubber molded articles that have good high-voltage electrical insulating characteristics and a high mechanical strength following molding.